The effects of the chemistry of ore-forming fluids on the sulfur and carbon isotopic compositions of hydrothermal minerals are quantitatively evaluated from available thermochemical data and isotopic fractionation factors.The isotopic composition of both sulfur and carbon in hydrothermal minerals is strongly controlled by the f (sub O 2 ) and pH values of hydrothermal fluids as well as by the temperature and the isotopic composition of sulfur and carbon in the fluids (delta S 34 (sub Sigma S) and delta C 13 (sub Sigma C) values). For example, at 250 degrees C and within geologically important f (sub O 2 ) -pH regions, an increase in f (sub O 2 ) value by 1 log unit or in pH by 1 unit can cause a decrease in delta S 34 values of sulfur-bearing minerals by as much as 20 per mil. An increase in f (sub O 2 ) by 1 log unit or in pH by 2 units can cause a decrease of about 30 per mil in delta C 13 values of carbon-bearing minerals. Large variation in the delta S 34 values or in the delta C 13 values of hydrothermal minerals, which often have been interpreted as an indication of biogenic sulfur or carbon, could also be caused by slight variation in the f (sub O 2 ) and/or pH of ore-forming fluids during ore deposition.The concentrations in an ore solution of sulfur (or f (sub S 2 ) ) and of carbon (or f (sub CO 2 ) ) place limits on possible delta S 34 and delta C 13 values for hydrothermal minerals. Sulfur-bearing minerals and carbon-bearing minerals precipitating from sulfur- and carbon-rich solutions can have wider ranges of delta S 34 and delta C 13 values than those minerals precipitating from sulfur- and carbon-poor solutions.Sulfide minerals which precipitated in equilibrium with magnetite, hematite, or sulfate minerals, and carbonate minerals which precipitated in equilibrium with graphite, could exhibit isotopic compositions markedly different from those of the depositing fluids. Therefore, sulfides with delta S 34 values near zero per mil or carbonates with delta C 13 values near -6 per mil do not necessarily indicate a magmatic origin for the sulfur or the carbon.The mode of variation on the delta S 34 values of sulfide minerals and in the delta C 13 values of carbonate minerals in a given deposit may indicate the relative oxidation states of ore-forming fluids: variable delta S 34 + uniform delta C 13 values may suggest that the minerals were precipitated under relatively high f (sub O 2 ) conditions; uniform delta S 34 + uniform delta C 13 values, under intermediate f (sub O 2 ) conditions; and uniform delta S 34 + variable delta C 31 values suggesting deposition under relatively low f (sub O 2 ) conditions.Sulfur and carbon isotopic data, combined with geological and mineralogical data of ore deposits, may define the physico-chemical parameters (T, f (sub O 2 ) , f (sub S 2 ) , f (sub CO 2 ) , m (sub Sigma S) , m (sub Sigma C) ) and the origin (delta S 34 (sub Sigma S) and delta C 13 (sub Sigma C) values) of ore-forming fluids as well as the mechanisms of ore deposition.